Effect of biochar and biodigester effluent on growth of maize in acid soils

Lylian Rodríguez, Patricia Salazar and T R Preston

Abstract

The
hypothesis that was tested in the present study was that there would be a
synergistic response in growth of maize when biodigester effluent, rich in NH4-N,
was combined with biochar, derived by gasification of sugar cane bagasse.
Two experiments were carried out to measure changes
in soil fertility as a function of the growth of maize plants over a 30-40 day
period following seeding. In each experiment a completely randomized
design was used with 4 replications of the treatments applied to samples of soil
held in one litre capacity plastic bags. In experiment 1, 8 treatments were
compared in a 2*2*2 factorial arrangement. The factors were: with or without
biochar at 50g/kg soil; fertile soil or sub-soil; and with or without
biodigester effluent (100 kg N/ha). In experiment 2, ash from a wood-burning
stove replaced the biochar used in experiment 1.

Biochar increased green biomass
growth of the maize on the fertile soil in absence or presence of biodigester
effluent and in the sub-soil when effluent was applied, but had no effect on
heavily leached soil without effluent. Application of effluent had no
effect on green biomass growth in the fertile soil irrespective of the presence
or not of biochar. By contrast, the effluent dramatically increased green
biomass growth when biochar was applied to the sub-soil but had no effect in the
absence of biochar. Effects on growth of the roots mirrored those on the green
biomass except in the case of the sub-soil without effluent when the biochar
markedly increased root growth. Soil pH was increased from 4-4.5 to 6.0-6.5 due
to addition of biochar. Wood ash brought about increases in the
weight of both the aerial part and roots of the maize but the relative increases
were only half of those observed when biochar was used. Soil pH was increased
to values between 9 and 10.

It is concluded that there are
synergistic effects on plant growth in heavily leached, acid soils when
biodigester effluent is combined with biochar produced by gasification of sugar
cane bagasse.

Introduction

Recent interest in the use of biochar as
a soil amender (Lehmann et al 2006; Lehmann 2007) has its origin in the discovery, by Dutch soil
scientist Wim Sombroek in the 1950's, of pockets of rich, fertile soil in the
Amazon rainforest (otherwise known for its poor, thin soils). He gave it the
name of
Terra Preta ("black earth"). Carbon dating has shown that the carbon
in these soils dates back to between 1,800 and 2,300 years (Glaser 2007).

Terra preta . is
rich in minerals including potassium, phosphorus, calcium, zinc, and manganese -
however it’s most important ingredient is charcoal, the source of terra
preta's
dark colour. The exact origin of the charcoal in Terra preta is not fully
understood but it appears to have arisen from controlled burning of trees and
related biomass sources. The fact that it has remained in the soil for
thousands of years implies that it can be an effective medium for long term
sequestration of carbon derived originally from the atmosphere through
photosynthesis. This also indicates that the form of the charcoal in Terra
preta soils is different to the charcoal prepared in the traditional manner
as a source of fuel for cooking. This has given rise to the term “biochar” to
differentiate this “stable” form of charcoal, that is not oxidized by soil
micro-organisms, as compared with charcoal which eventually is degraded by
soil microbes to carbon dioxide. According to Glaser (2007) the
chemical structure of biochar is characterized by the presence of poly-condensed
aromatic moieties and that these are responsible for the stability against
microbial degradation.

The apparent
high fertility of Terra preta soils,, has led to research to
measure the immediate effects of “biochar” addition to soils on plant
growth. Major increases
(up to 324%) in yield of a range of crops through addition of biochar at
rates varying from 0.5 to 135 tonnes/ha were recorded in the review by Sohi
et al (2009). However,
these authors state that addition of nutrients
from inorganic or organic fertilizers is usually essential for high productivity
and to increase the positive response from the bio-char amendment. Chan et al
(2008)
recorded a linear increase in yield of radish (Raphanus sativus)
by addition of up to 50 tonnes/ha of biochar provided additional N fertilizer
was also supplied. Glaser (2007) also indicated that there would be benefits in
plant growth from combining the biochar with chicken manure.

The explanations for the effects of addition of biochar to soils in increasing
crop yields include greater water holding capacity, increased Cation Exchange
Capacity (CEC), and providing a medium for adsorption of plant nutrients and
improved conditions for soil micro-organisms (Sohi et al 2009). Biochar efficiently
adsorbs ammonia (NH3)
according to Oya and Iu (2002) and Iyobe et al (2004) and acts as a binder
for ammonia in soil, therefore having the potential to decrease ammonia
volatilization from soil surfaces .

Biodigestor effluent from live stock excreta contains a high proportion of the
nitrogenous constituents as ammonium salts. Pedraza et al (2002) observed
that the proportion of ammonia-N in the effluent from plug-flow, tubular plastic
biodigesters was in the range of 0.65 to 0.75. Similar findings were reported by
San Thy et al (2003). In their study, the proportion of ammonia-N to
total-N increased from 0.077 to 0.12 in fresh pig manure to 0.46 to 0.65
in the effluent. The combination of biodigester effluent and biochar therefore
should be synergistic in improving soil fertility and plant growth

The hypothesis that
was tested in the present study was that there would be a synergistic response
in growth of maize when biodigester effluent is combined with biochar.

Materials and methods

Location

The study was carried out in the "Finca Ecológica", TOSOLY,
Morario, Guapota, Department of South Santander, Colombia (6° 18" N, 73° 32" W,
1500 msl) between September and December 2008. Air temperature ranges between 19
and 28°C in the day, falling to around 12°C during the night. Rainfall is
between 2700 and 3000 mm/year and is relatively evenly distributed.

Treatments and design

Two experiments were carried out using the maize “biotest” for measuring changes
in soil fertility as a function of the growth of maize plants over a 30-40 day
period following seeding (Boonchan Chantaprasarn and Preston (2004). In
each experiment a completely randomized design was used with 4 replications of
the treatments applied to samples of soil held in one litre capacity plastic bags (Photo 1).

Photo 1.
General view of the layout of the “biotest”

Experiment 1: Effect of
biochar added to soil (pH 4.0) at 50 tonnes/ha with and without biodigester
effluent (100 kg N/ha) on growth of maize

Eight treatments were compared in a 2*2*2 factorial arrangement with 4
replications.

The factors were:

Biochar: With or without biochar

Soil type: Fertile soil or sub-soil

Biodigester effluent: With or without
effluent

Experiment 2: Effect of wood ash added to soil at 50
tonnes/ha with or without biodigester effluent

The eight treatments and the design were
the same as in Experiment 1 but with wood ash replacing the biochar. The factors
were:

Wood ash: With or without

Soil type: Fertile soil or sub-soil

Biodigester effluent: With or without

Materials

Biochar

The biochar (Photo 2) was the solid residue from a down-draft gasifier (Photo 3;
Ankur PTY, India), charged with sugar cane bagasse derived from sugar cane
stalks passed two times through a 3-roll crusher driven by a diesel engine
(Photo 5). It contained 35% ash and 65% carbon and had a pH of 9.0. The bagasse
was sun-dried to about 12% DM and hand-separated into large and small pieces,
the latter being the feedstock for the gasifier. The particle size of this
fraction was between 1 and 30mm (Photo 4). After gasification of this fraction
the residual biochar represented 10% by weight of the air-dry bagasse (88% DM)
fed into the gasifier.

Photo 2. Biochar produced by
gasification of sugar cane bagasse

Photo 3.
T he downdraft gasifier (Ankur Technologies) used to produce the
biochar as a byproduct of electricity generation

Photo 4.
Sugar cane bagasse used in the gasifier

Photo 5.
The three-roll crusher (“trapiche”) used to fractionate the sugar
cane stalks into juice (for feeding pigs and people) and residual
bagasse

Wood ash

This was the residue after burning
firewood in a closed stove (Photo 6). The pH was 9.5.

Soil samples

Two types of soil were used in each
experiment. The fertile soil was taken from areas (top 10cm) in a coffee
plantation shaded with Guamo trees (Inga hayesii Benth) (Photo 7). The pH
of this soil was 4.5.
The su-soil; second sample (sub-soil) was from soil that had been
excavated during construction work (Photo 8). The pH was 4.0.

Samples (about 1 kg) of the respective
soils were placed in polyethylene bags with or without addition of the biochar
(or ash) which was mixed thoroughly with the soil according to the imposed
treatment. Water was sprayed uniformly on the bags at 2-day intervals throughout
the growth period of 40 days.

Photo 6.
The wood stove used to produce the ash used in Experiment 2

Photo 7.
The coffee plantation
from where fertile soil was taken

Photo 8.
The origin of the sample
of sub-soil

Photo 9.
The plug-flow tubular
polyethylene biodigester

Biodigester effluent

The effluent was taken from the exit stream of a
“plug-flow” tubular polyethylene (3.0 m3 liquid volume) biodigester
(Photo 9) charged daily with the washings (500 litres) from 4 pens each holding
on average 8 pigs of 50 kg mean live weight. The diet of the pigs (DM basis) on
average contained 20% soybean meal, 30% rice polishings and 50% fresh sugar cane
juice. The N content of the effluent was 700 mg/litre with 420 mg/litre as NH4-N.
It was poured on the surface of the soil in the bags at weekly intervals (5
applications) at the overall rate of 100 kg N/ha. Similar amounts of water were
added to the bags not receiving effluent.

Maize seeds

These were of a local variety. Three seeds were placed in each bag. After
germination, one or two seedlings were removed to leave only one plant for the
experimental growth period of 40 days.

Measurements

At 40 days after seeding the height of the maize was measured at the tip of the
highest leaf. The complete plant was then removed from the bag and the aerial
part separated from the roots which were washed free of soil. Both fractions
were weighed. The pH of the soil was measured with a digital pH meter at
the time of harvesting the maize.

Statistical analysis

The data were analysed using the General Linear Model
in the ANOVA option of the Minitab (2003) software. Sources of variation were:
Blocks, Biochar, Effluent, Soil type, and the interactions of Biochar*Effluent,
Biochar*Soil, Effluent*Soil, Biochar*Effluent*Soil and error.

Results

Effects of biochar

The height and the fresh weights of the
aerial part and the roots of the maize were increased by addition to the soil of
biochar and biodigester effluent and were higher for the maize grown in the
fertile compared to the sub-soil (Table 1). Soil pH was increased by
addition of biochar and was higher in the fertile soil.

Table 1. Mean
values for effects of biochar, effluent and soil type on height and
fresh weights of aerial part and roots of maize, and on soil ph
(after 40 days growth of the maize)

Height, cm

Aerial part, g

Roots, g

Soil pH

Biochar

With

53.4

30.3

38.4

6.33

Without

27.1

5.78

10.1

4.58

P

0.001

0.001

0.001

0.001

Effluent

With

48.0

25.9

30.4

5.43

Without

32.6

10.1

18.2

5.48

P

0.002

0.001

0.012

0.25

Soil

Fertile

50.7

23.3

30.5

5.73

Heavily leached

29.9

12.8

18.1

5.17

P

0.001

0.006

0.001

0.001

SEM

3.13

2.41

3.13

0.045

P (interactions)

B*E

0.14

0.005

0.005

0.27

B*S

0.85

0.66

0.66

0.001

E*S

0.04

0.075

0.075

0.166

B*E*S

0.03

0.011

0.095

0.83

There were interactions due to the treatments on
weights of aerial part and roots of the maize (Table 1 and Figures 1 and
2) for biochar*effluent and biochar*effluent*soil type with tendencies for
interaction (P=0.075) for effluent*soil.

Figure 1. Effect of
biochar and effluent added to fertile soil and sub-soil
on fresh weight of aerial part of maize (40 days of growth)

Figure 2. Effect of
biochar and effluent added to fertile soil and sub-soil
on fresh weight of maize roots (40 days of growth)

Biochar increased green biomass growth of the maize
on the fertile soil in absence or presence of biodigester effluent and in the
sub-soil when effluent was applied, but had no effect on sub-soil without
effluent (Figure 1). Application of effluent had no effect on green
biomass growth in the fertile soil irrespective of the presence or not of
biochar. By contrast, the effluent dramatically increased green biomass growth
when biochar was applied to the sub-soil but had no effect in the absence of
biochar. Effects on growth of the roots mirrored those on the green biomass
except in the case of the sub-soil without effluent when the biochar markedly
increased root growth (Figure 2).

Soil pH was increased by nearly 2 units due to addition of biochar (Table 1 and
Figure 3). There was no effect on soil pH due to application of effluent
but values were 0.5 pH units higher on average for the fertile soil compared
with the sub-soil.

Figure 3.
Effect of biochar and effluent on soil pH in fertile soil and
sub-soil

Effects of wood ash

In view of the major increase in soil pH
(from 4-4.5 to 6.0-6.5) caused by the addition of biochar, it was hypothesized
that one reason for the stimulatory effect of biochar on growth of maize might
have been caused by the increase in soil pH.

Experiment 2 was designed to study the
effect of ash per se in the absence of the carbon which is the other
major component of the biochar used in this study. The addition of the wood ash,
admittedly at ash levels some 30% higher than when biochar was used, brought
about increases in the weight of both the aerial part and roots, of the maize
but the relative increases were much less than when biochar was used (Table 2;
Figures 4 and 5).

Table 2. Mean
values for effects of wood ash, effluent and soil type on height and
fresh weights of aerial part and roots of maize, and on soil ph
(after 40 days growth of the maize)

Height, cm

Aerial part, g

Roots, g

Soil pH

Wood ash

With

36.9

10.6

11.5

9.49

Without

22.3

2.89

4.34

4.4

P

0.003

0.001

0.02

0.001

Effluent

With

29.4

7.40

7.51

6.97

Without

29.8

6.05

8.30

6.92

P

0.92

0.48

0.78

0.25

Soil

Fertile

39.1

10.6

10.0

6.65

Heavily leached

20.1

2.86

5.84

7.24

P

0.001

0.001

0.16

0.001

SEM

3.1

1.3

2

P (interactions)

B*E

0.14

0.005

0.005

0.27

B*S

0.85

0.66

0.66

0.001

E*S

0.04

0.075

0.075

0.166

B*E*S

0.03

0.011

0.095

0.83

Figure 4. Effect of
wood ash and effluent added to fertile soil and sub-soil on
fresh weight of aerial part of maize (40 days of growth)

Figure 5. Effect of
wood ash and effluent added to fertile soil and sub-soil on
fresh weight of the roots of maize (40 days of growth)

Soil pH was raised
by 5-6 pH units (Figure 6) to values between 9 and 10. This high degree of
alkalinity may have been a deterrent to plant growth.

Figure 6.
Effect of wood ash and effluent on soil pH in fertile soil and sub-soil

Discussion

The 10% yield of biochar from gasification of sugar
cane bagasse is similar to values reported by Miech Phalla and Preston (2005) for
Mulberry stems (10.9%), Cassava stems (12.8), Coconut shells (13.7%) and
branches from the leguminous tree
Cassia stamea (10.9%), processed in a similar model of gasifier (Ankur PTY,
India).

The increase in growth of the maize brought about by the biochar is in agreement
with the majority of reports in the literature (see Sohi et al 2009). Two
factors appear to distinguish the biochar used in these studies and that used in
most reported experiments. First, the biochar was the product of gasification
and therefore would have been submitted to higher temperatures than biochar
derived by pyrolysis; and secondly it had a very high content of ash. Such a
high ash content (35%) in the biochar used in these studies has not apparently
been observed in other experiments. In the research reported by Rondon et al
(2007) the biochar was made by pyrolysis of eucalyptus logs and contained only
0.3% of ash. Their data showed an increase in soil pH from 5.0 to 5.4 after
applying 40g biochar per 1 kg of soil, much less than the increase from 4.6 to
6.3 in our experiment. The results from using wood ash as soil amendment in
Experiment 2 were confounded by the higher level (of ash) that was used and the
resulting major increase in soil pH (from 4.4 to 9.5). The much lower growth
response of the maize to the wood ash compared with the biochar could be
interpreted as the consequence of the absence of the carbon and related organic
compounds in the biochar, and/or the negative effect of the excessive alkalinity
(soil pH of 9.5) which would have reduced phosphorus availability with formation
of insoluble calcium phosphate.

Many researchers have emphasized the importance of nutrient supply, especially
nitrogen, as a determinant of plant growth response to soil amendment with
biochar (see review by Sohi et al 2009). The significant interaction between
application of biodigester effluent and biochar in the sub-soil, but not in the
fertile soil, confirms the importance of the relationship between nutrient
supply and response to biochar. These findings emphasize the major benefits that
biochar combined with biodigester effluent can confer on poor soils with little
or no organic matter and low nutrient status (Photos 10 and 11). Similar
synergistic effects on plant growth by combining charcoal with chicken manure
were observed by Steiner et al (2007).

Photo 10. The
sub-soil with no biochar or effluent

Photo 11. The
sub-soil after amendment with biochar and effluent

Conclusions

Biochar produced as a byproduct
of the gasification of sun-dried, sugar cane bagasse (the cane stalks were
passed two times through a 3-roll mill traditionally employed for making
“panela”), contained 35% ash.

Application of the biochar (50
g/kg of soil) to a fertile soil (from a shaded coffee plantation) increased
above ground biomass growth five-fold with no additional benefit from
simultaneous application of biodigester effluent. When applied to a
sub-soil, there was a synergistic effect of the biochar and the biodigester
effluent; the biochar alone increased yield eight-fold but combined with
biodigester effluent the increase was twenty-fold. Effects on the root
biomass were similar.

The initial pH of both soils was
in the range of 4.0-4.5 and was increased to 6.0-6.5 by addition of the
biochar. Effluent application did not affect soil pH.

Application of ash from a
wood-burning stove at 50g/kg soil also increased maize yield but to a level
of only one third of that achieved with biochar. The increase in soil pH was
double that observed with biochar reaching levels of between 9 and 10.

San Thy, Preston T R and Ly J
2003 Effect of
retention time on gas production and fertilizer value of biodigester effluent.
Livestock Research for Rural Development 15 (7). http://www.lrrd.org/lrrd15/7/sant157.htm